1. Field of the Invention
The present invention concerns a silicon melting crucible used for pulling silicon single crystal and, more specifically, it relates to a silicon melting crucible for preventing deformation of an upper portion of a crucible during pulling.
2. Description of Related Art
For manufacturing silicon single crystals, a method of melting a starting silicon material at high purity in an argon atmosphere under a reduced pressure and solidifying the same using a seed crystal while pulling upwardly by a Czochralski method is known.
FIG. 1 is a view illustrating a silicon single crystal manufacturing device for solidifying a starting molten liquid while pulling up a seed crystal by using a Czochralski method. A pulling up atmosphere for silicon single crystal is tightly sealed in a chamber 6, a crucible 1 is disposed at the inside of the chamber, a heating heater 4 comprising, for example, induction heating coils is disposed at the outer side of and surrounding the crucible, and a heat insulating cylinder 5 formed cylindrically by a heat insulating material is disposed further to the outside thereof. A material for forming crystals melted by the heating heater 4, that is, a molten liquid 7 of the silicon starting material is contained in the melting crucible 1. A lower end of a seed crystal 8 attached to the top end of a pulling rod or wire 10 is in contact with the surface of the molten liquid and, when the seed crystal is pulled upwardly, a silicon single crystal 9 solidified from the molten liquid is grown to the lower end
The silicon melting crucible 1 is supported and rotated by a rotational shaft 13, while the silicon single crystal 9 is driven by a rotational mechanism (not illustrated) disposed at an upper portion of the pulling rod and they are rotated in the directions opposite to each other. Further, an argon gas is supplied from a gas supply port 11 and discharged from a gas discharging port 12.
The melting crucible 1 used has a double structure in which the inner wall comprises a container 2 made of quartz (hereinafter referred to as "quartz crucible") while the outer wall comprises a container 3 made of carbon (hereinafter referred to as "carbon crucible"). When the silicon single crystal is manufactured, the diameter of the quartz crucible 2 is enlarged as the pulling diameter is increased, the amount of heat calories used for heating is also increased and the surface temperature of the crucible is also made higher.
Usually, the upper end of the melting crucible is constituted such that the side wall of the quartz crucible is higher than the side wall of the carbon crucible. Therefore, when the crucible is exposed to a high temperature during pulling of the silicon single crystal, the quartz crucible is deformed by turning-down at the upper portion to the inner side, or buckling may be caused by the weight of the quartz crucible. Upon such an incident, pulling up of silicon single crystal has to be interrupted. Further, during manufacture of the silicon single crystal, since a SiO gas is released always from the surface of the molten liquid, the SiO gas invades between the quartz crucible and the carbon crucible, by which carbon (C) is involved in a siliciding reaction (SiO+C.fwdarw.SiC) to lower the working life of the carbon crucible.
FIG. 2 and FIG. 3 are views illustrating the state for the deformation of the quartz crucible and occurrence of the siliciding reaction to the carbon crucible during pulling up of the silicon single crystal. FIG. 2 is a vertical cross sectional view of a melting crucible illustrating a state in which the upper portion of a quartz crucible of the silicon melting crucible is turned-down to the inside (on the left of the figure) and a state in which siliciding reaction occurs to the carbon crucible (on the right of the figure). Further, FIG. 3 is a vertical cross sectional view of the melting crucible illustrating a state of buckling (on the left of the figure) and a state of bending (on the right of the figure) caused by the weight of the quartz crucible of the silicon melting crucible.
FIG. 2 shows a turned-down portion 18 in which the upper portion of the quartz crucible is deformed so as to turn-down to the inside of the crucible on the left, while showing a portion 19 causing a siliciding reaction to the carbon crucible on the right. As shown on the left of the figure, if the turned-down portion 18 is formed on the quartz crucible, the SiO gas is liable to be deposited on that portion. As the deposition continues, the SiO gas is oxidized into granular SiO.sub.2 and falls into the molten silicon liquid.
While the single crystal during pulling is pulled up in a dislocation free state, fallen SiO.sub.2 moves to the growing boundary of the silicon single crystal and then invades into the single crystal thereby giving rise to dislocation and requiring interruption of pulling. Further, if the portion 19 causing the siliciding reaction is present in the carbon crucible as shown on the right of the figure, a gap is formed between the carbon crucible and the quartz crucible and the SiO gas further intrudes to silicide the carbon crucible more and results in severe injury making it impossible for reuse.
FIG. 3 shows a buckled portion 20 caused by the weight of the quartz crucible on the left, while a bent portion 21 caused by softening of the quartz crucible under a high temperature on the right of the figure. Also in case of deformation of the quartz crucible, pulling of dislocation free crystal is difficult by the deposition of SiO and falling of granular SiO.sub.2 like that in the turned-down deformation shown in FIG. 2.
For overcoming such problems, Japanese Laid-open Patent Application No. 63-315263 proposes a melting crucible in which a flange diverging outwardly is provided integrally to the upper end of a cylindrical side wall of a quartz crucible. However, fabrication of the integrally disposing outwardly diverging flange is troublesome making the cost higher. Further, Japanese Laid-open Patent Application No. 3-290393 has proposed a melting crucible in which a quartz ring is disposed on and covers the upper end of a cylindrical side wall of a melting crucible. However, when exposed to a high temperature, the quartz ring is fused to the quartz crucible and is no more detachable therefrom to result in a problem that the quartz ring can not be utilized again.
Further, since it is important to enable high speed pulling and thereby improving the productivity in the manufacture of the silicon single crystal, Japanese Laid-open Patent Application No. 6-32692 proposes a silicon melting crucible in which the upper end of a side wall of a quartz crucible is tapered so as to diverge upwardly in order to increase the amount of polycrystal silicon to be filled as the starting crystal material and it is described that a tapered angle of 45.degree. at the upper end of the side wall is particularly suitable. However, the proposed quartz crucible intends to increase only the filling amount of the starting crystal material and takes no consideration for preventing deformation such as turning-down and buckling of the quartz crucible or generation of siliciding reaction to the carbon crucible at all. For instance, if the silicon single crystal pulling is practiced under the condition at the tapered angle of 45.degree. adopted there, the upper portion of the quartz crucible deforms remarkably toward the inside of the crucible to form a bent portion 21 as shown on the right of FIG. 3. As a result, flow of an argon gas in the single crystal pulling region is disturbed, to bring about a problem that the SiO gas generated from the surface of the molten liquid can not be discharged efficiently, but granular SiO.sub.2 deposited on the deformed portion falls into the molten liquid to give rise to dislocation for the single crystal to be manufactured.